As is well known, gas-operated auto-loading firearms use a portion of the projectile propellant gases to cycle the firearm action, extracting and ejecting the spent casing and automatically loading another cartridge into the barrel chamber. A small portion of the propellant gas pressure is diverted through a port from the barrel bore and through a gas block mounted on the barrel. In a direct impingement system, a flow of gas pressure is conveyed directly into the bolt carrier to cycle the action. In gas piston systems, the gas is used to actuate a piston that is either directly (long stroke) or indirectly (short stroke) connected to the bolt carrier assembly by an operating rod.
In some known gas piston systems, after the piston has been actuated, the remaining gas pressure may not be vented, in which case it returns through the barrel gas port into the barrel bore and exhausts with the rest of the propellant gas pressure through the muzzle. In other systems, after the piston has been displaced a preselected distance, gas may be vented through an exhaust port in the cylinder wall. In some cases, this results in venting of hot propulsion gases inside a forearm or hand guard, causing a significant transfer of heat to an area where the user commonly places a hand for supporting the firearm. Alternatively, other systems have provided a forwardly directed gas exhaust vent, but these systems have required complex machining to manufacture the parts, have been difficult or cumbersome to disassemble, and/or have been difficult to thoroughly clean.